Electronic display systems are commonly used to display information, especially information containing motion or providing interactivity. Typical display systems range in size from small displays used in mobile devices to very large displays visible to thousands of viewers. In general, larger displays are more expensive than smaller displays. Moreover, very large flat-panel displays (e.g. greater than approximately 3 meters in diagonal) cannot be made on a monolithic substrate. Such very large displays are typically made by either positioning individual point-source light-emitting diodes into a large backplane or by tiling smaller displays together. For example, video walls using multiple video displays are frequently seen in the electronic media. Alternatively, multiple projector systems used to create a large, tiled, high-resolution display are also available. Tiled flat-panel displays including a plurality of liquid crystal displays are also known. However, such displays typically have visible seams between the tiles, resulting in less than satisfactory image quality.
Various approaches to solving the problem of tile seams are described in the art. U.S. Pat. No. 5,838,405 entitled “Tiled Display Device” describes adhering multiple, adjacent LCD displays between a large substrate and a flexible coat layer. An adhesive agent is placed between the displays and both the large substrate and flexible coat layer which ideally has an optical matched to the large substrate, flexible coat layer, and the LCD panels to prevent trapping of light within the structure. Further, as the large substrate and flexible coat layer are attached to the LCDs, the adhesive agent fills the gaps between adjacent LCDs, preventing reflecting or scattering of light within this structure. Polarizer sheets are provided on both sides of the laminated LCD structure with orthogonal linear polarization. A monolithic backlight is provided behind the entire structure. In this structure, the presence of the adhesive agent between the LCD panels, sandwiched between the large substrate and flexible coat layer, limits optical surfaces between the large substrate and the flexible coat layer which can reflect or refract the light. Therefore, this adhesive agent provides the function of preventing reflection or refraction of light from the backlight as it passes between the two polarizing sheets to prevent loss of polarization. However, this adhesive agent must be sandwiched appropriately between the flexible coat layer and large substrate to achieve this function. In this configuration, because the light is polarized by a first polarizer and unaffected as it passes through this structure, the second polarizer will effectively absorb the light that passes from the backlight and through the gaps between adjacent LCD panels such that no light is emitted in the areas between the LCD panels. The transparency of this material is not discussed as its primary purpose is to insure that none of the light emitted by the backlight in the vicinity of the area between the tiles is passed through the device and therefore, this material can be opaque. Also, the filler of this scheme is UV curable and thus it must be possible to pass light into the device to cure the adhesive agent, which is disposed on both sides of and between the LCD tiles.
U.S. Pat. No. 6,097,455 entitled “Flat Panel Displays having Tiles and Visually Imperceptible Gaps There Between” also describes an LCD tiled structure. Adhesive layers together with LCD tiles and other optical components are located between a back substrate and a front cover plate. As discussed in this patent, masking elements are positioned over the tile gaps to prevent light from passing from the backlight into the gaps between the LCD panels. Once again, this configuration prevents light emitted from a backlight from emerging between the LCD panel tiles.
OLED display devices are also becoming popular and are employed in tiled display systems. Light emitting diodes (LEDs) incorporating thin films of light-emitting materials forming light-emitting elements have many advantages in a flat-panel display device and are useful in optical systems. U.S. Pat. No. 6,384,529 to Tang et al. shows an organic LED (OLED) color display that includes an array of organic LED light-emitting elements. Alternatively, inorganic materials are employed and can include phosphorescent crystals or quantum dots in a polycrystalline semiconductor matrix. Other thin films of organic or inorganic materials can also be employed to control charge injection, transport, or blocking to the light-emitting-thin-film materials, and are known in the art. The materials are placed upon a substrate between electrodes, with an encapsulating cover layer or plate. Light is emitted from a pixel when current passes through the light-emitting material. The frequency of the emitted light is dependent on the nature of the material used. In such a display, light is emitted through the substrate (a bottom emitter) or through the encapsulating cover (a top emitter), or both.
The optical structures of LCD tiled display systems are typically not well-adapted to the optical structures of area-emissive electroluminescent (EL) devices, such as Organic Light Emitting Diode (OLED) devices, since the use of cross polarizers and liquid crystals modulators within confined light pathways is fundamentally different from the direct modulation and emission of light from EL displays. In tiled LCDs, the light is emitted from a backlight and emitted between panels without being modulated by the LCDs within the tiles. However, in EL devices light is emitted within each tile and will not be emitted within an area between the tiles. Further, in LCDs, light from a backlight is directed perpendicular to the substrate as it passes through LCD panels. However, the angles of light entering the substrate of an EL device typically have a Lambertian distribution, that is, light is emitted equally in every direction at a plane that is very near the substrate surface. The light emitted within an EL device tile thus enters the substrate of an OLED traveling in all directions. Because EL devices employ high-optical-index emissive materials, which emit light into the substrate according to a near Lambertian distribution, a large fraction (e.g. greater than 50%) of the emitted light is typically trapped in the device due to total internal reflection and thus reduces the device efficiency. In particular, approximately the same amount of light is trapped in the device substrate as is emitted from the device. This light travels through the substrate and is emitted from the edge of the substrate.
Tiled display systems employing EL devices, specifically OLED displays are also known. U.S. Pat. No. 6,498,592 entitled “Display Tile Structure Using Organic Light Emitting Materials” describes a tiled display structure fabricated on a single substrate that also serves as a circuit board. However, this design requires that the inter-pixel distance be the same within a tile as between tiles. Ricks et al in U.S. Patent Application Publication No. 2007/0001927 describe overlapping tile elements for display that can be expanded, collapsed, folded, and rolled. However, such overlap area requires that a tile substrate be flexible and non-planar. WO 2006/027727 describes a tile electronic interconnection method. However, none of these disclosures address the issue of light leakage between tiles.
U.S. Pat. No. 6,873,380 entitled “Providing Optical Elements over Emissive Displays” describes an EL display formed of a plurality of abutted tiles, each tile contributing a portion of the overall displayed image. In this patent, optical elements are selectively situated between pixels to improve the optical performance of the display and can facilitate thicker cover glasses over the display. The disclosure describes the use of a black strip to absorb light between the tiles. The presence of this black strip prevents viewing of light that is emitted through the edges of each tile. Unfortunately, when a display is equipped with such barriers and then viewed at an angle, the absorber can obscure pixels on either side of the black strip, producing a perceptible seam at those angles.
There is a need therefore for tiled EL system that improves the light emission uniformity of EL flat panel display systems such that the appearance of the EL device is consistent both within and between tiles. Ideally, such a device will also provide reduced reflection from ambient light.